Handle circular dependencies when getting extras (#2787)

Resolves: #2376

poetry/utils/extras.py

@@ -33,6 +33,9 @@ def get_extra_package_names(
         for extra_package_name in extras.get(extra_name, ())
     ]
 
+    # keep record of packages seen during recursion in order to avoid recursion error
+    seen_package_names = set()
+
     def _extra_packages(package_names):
         """Recursively find dependencies for packages names"""
         # for each extra pacakge name
@@ -41,11 +44,16 @@ def _extra_packages(package_names):
             # dependency (like setuptools), which should be ignored
             package = packages_by_name.get(canonicalize_name(package_name))
             if package:
-                yield package.name
+                if package.name not in seen_package_names:
+                    seen_package_names.add(package.name)
+                    yield package.name
                 # Recurse for dependencies
                 for dependency_package_name in _extra_packages(
-                    dependency.name for dependency in package.requires
+                    dependency.name
+                    for dependency in package.requires
+                    if dependency.name not in seen_package_names
                 ):
+                    seen_package_names.add(dependency_package_name)
                     yield dependency_package_name
 
     return _extra_packages(extra_package_names)

tests/utils/test_extras.py

@@ -9,6 +9,12 @@
 _PACKAGE_BAR = Package("bar", "0.3.0")
 _PACKAGE_BAR.add_dependency("foo")
 
+# recursive dependency
+_PACKAGE_BAZ = Package("baz", "0.4.0")
+_PACKAGE_BAZ.add_dependency("quix")
+_PACKAGE_QUIX = Package("quix", "0.5.0")
+_PACKAGE_QUIX.add_dependency("baz")
+
 
 @pytest.mark.parametrize(
     "packages,extras,extra_names,expected_extra_package_names",
@@ -40,6 +46,12 @@
             ["group0", "group1"],
             ["bar", "foo", "spam"],
         ),
+        (
+            [_PACKAGE_BAZ, _PACKAGE_QUIX],
+            {"group0": ["baz"], "group1": ["quix"]},
+            ["group0", "group1"],
+            ["baz", "quix"],
+        ),
     ],
 )
 def test_get_extra_package_names(